Semiconductor lasers have many advantageous features, such as smallness, inexpensiveness, low power consumption and long service life, and they are used in a wide variety of fields, such as light sources for optical recoding, light sources for communication, laser displays, laser printers and laser pointers. In normal types of laser displays or laser printers, the laser beam is controlled to scan a certain area so as to create characters or figures. In currently used semiconductor lasers, the scan operation is achieved by controlling the emitting direction of the laser beam by an additional, external element, such as a polygon mirror, MEMS (micro-electro mechanical system), micro mirror or acousto-optic device. However, adding such a scanning mechanism makes it difficult to create a semiconductor laser that is smaller in size, quicker in operation and higher in durability.
Based on this point of view, the present inventors have created a two-dimensional photonic crystal surface emitting laser whose beam-emitting direction is variable (Patent Document 1). This device is hereinafter called the “variable beam-direction two-dimensional photonic crystal surface emitting laser.”
Initially, a normal type of two-dimensional photonic crystal surface emitting laser (which itself does not have a variable emitting direction) is hereinafter described as a basis for understanding the variable beam-direction two-dimensional photonic crystal surface emitting laser. Two-dimensional photonic crystal surface emitting lasers normally have an active layer and a two-dimensional photonic crystal layer consisting of a plate-shaped member in which modified refractive index areas whose refractive index differs from that of the plate-shaped member (and which consist of air holes or areas made of a material whose refractive index differs from that of the plate-shaped member) are periodically arranged. In this two-dimensional photonic crystal surface emitting laser, when electric charges are injected into the active layer, rays of light are generated within a wavelength range determined by the material of the active layer. Among the generated light, the light having a wavelength equal to a predetermined “in-medium wavelength” determined by the spatial period of the modified refractive index areas (i.e. the wavelength of light within the two-dimensional photonic crystal layer, which equals the wavelength of light in vacuum divided by the average refractive index of the two-dimensional photonic crystal layer) forms a standing wave, whereby a resonant state of light is created. For example, if the modified refractive index areas are arranged in a square lattice pattern, the resonant state of light is created when the in-medium wavelength coincides with the spatial period of the modified refractive index areas.
The light which has caused the resonation is scattered by the modified refractive index areas in various directions within the two-dimensional photonic crystal. When two rays of light respectively scattered by two neighboring modified refractive index areas in the direction perpendicular to the two-dimensional photonic crystal layer have an optical path difference equal to their wavelength, these rays of scattered light will be in phase. For example, this condition is satisfied if the modified refractive index areas are arranged in a square lattice pattern and the in-medium wavelength coincides with their spatial period. The rays of light which are in phase with each other and scattered in the perpendicular direction form a laser beam emitted in the direction perpendicular to the two-dimensional photonic crystal layer.
The variable beam-direction two-dimensional photonic crystal surface emitting laser described in Patent Document 1 has an active layer and a pair of two-dimensional photonic crystal layers which differ from each other in the spatial period of the modified refractive index areas. In this device, among the light generated in the active layer, two rays of light having different wavelengths which respectively correspond to spatial periods of the refractive index distributions of the two layers form standing waves, thus creating a resonant state in both of the two-dimensional photonic crystal layers. Due to the frequency difference between the two standing waves, a spatial beat occurs, causing the resulting laser beam to be inclined from the direction normal to the two-dimensional photonic crystal layers. Such an obliquely emitted laser beam is hereinafter called the “inclined beam.” The angle (inclination angle) of the inclined beam from the normal to the two-dimensional photonic crystal layers increases as the aforementioned frequency difference increases. Therefore, by providing at least one of the two-dimensional photonic crystal layers with a refractive-index distribution whose spatial period varies depending on the in-plane position, it is possible to generate an inclined beam whose inclination angle changes depending on the position at which electric charges are injected into the active layer (i.e. the in-plane position where the laser oscillation occurs).
Providing the two-dimensional photonic crystal surface emitting laser with such an internal function of controlling the emitting direction of the inclined beam not only allows the devices in the aforementioned conventional application fields using semiconductor lasers to be smaller in size, quicker in operation and higher in durability, but also opens the possibility of creating new application areas, such as a mobile laser display, an inter-chip optical communication, or a laser knife to be incorporated in a capsule endoscope.